It is known to fabricate thermal IR detectors on a silicon substrate consisting of a thin membrane layer (made of electrically insulating layers) that is formed by etching of part of the substrate. Incident IR radiation increases the temperature of the membrane—which can be measured by either a thermopile, a resistor, or a diode.
For Example, Schneeberger et. al “Optimized CMOS Infrared Detector Microsystems,” Proc IEEE Tencon 1995, reports fabrication of CMOS IR detectors based on thermopiles. The thermopile consists of several thermocouples connected in series. KOH is used to etch the membrane and improve the thermal isolation. Each thermocouple consists of 2 strips of different materials, connected electrically and forming a thermal junction at one end (termed hot junction) while the other ends of the material are electrically connected to other thermocouples in series forming a thermal cold junction. The hot junctions of the thermocouples are on the membrane, while the cold junction is outside the membrane. Three different designs of the thermocouples are given in the paper with different material compositions: Aluminium and p-doped polysilicon, Aluminium and n-doped Polysilicon, or p-doped polysilicon and n-doped polysilicon. Incident IR radiation causes a slight increase in temperature of the membrane. The Seebeck effect causes a slight voltage difference across each thermocouple—resulting in a much large increase in voltage difference across the thermopile which is the sum of the voltages across each thermocouple.
Previously, Nieveld “Thermopiles Fabricated using Silicon Planar Technology,” Sensors and Actuators 3 (1982/83) 179-183, showed the fabrication of a thermopile on a micro-chip based on aluminium and single crystal silicon P+ as the materials in the thermocouple. It should be noted that this was a general thermopile device—not intended for IR detection and the thermopile was not on a membrane.
Allison et. al, “A bulk micromachined silicon thermopile with high sensitivity,” Sensors and Actuators A 104 2003 32-39, describes a thermopile based on single crystal silicon P-doped and N-doped materials. However, these are formed by waferbonding of a P-type wafer and an N-type wafer and is also not specifically for use as an IR detector. The fabrication method is also very expensive.
Lahiji et. al., “A Batch-fabricated Silicon Thermopile Infrared Detector,” IEEE Transactions on Electron Devices” 1992, describe two thermopile IR detectors, one based on Bismuth-antimony thermocouples, and the other based on polysilicon and gold thermocouples.
U.S. Pat. No. 7,785,002 describes an IR detector with a thermopile based on P and N doped polysilicon. Langgenhager “Thermoelectric Infrared Sensors by CMOS Technology,” IEEE EDL 1992, describes IR detectors consisting of thermopiles on a suspended structure consisting of aluminium and polysilicon.
Several other Thermopile devices are described by Graf et. al. “Review of micromachined thermopiles for infrared detection,” Meas. Sci. Technol. 2007.
Another method of measuring the IR radiation is by the use of thermodiodes. For example, Kim “A new uncooled thermal infrared detector using silicon diode,” S&A A 89, 2001, describes a diode fabricated by micromachining for use as an IR detector.
Eminoglu “Low-cost uncooled infrared detectors in CMOS process,” S&A A 109 (2003), describes IR detectors made using a CMOS process with diodes on a suspended membrane.
Similarly thermodiode based IR detectors can also be made using an SOI process. However, thermodiodes have the disadvantage that they need a biased voltage or current—which requires power. In addition, it has a high base voltage, which makes it harder to measure small changes in the output voltage.
It is also known to fabricate IR sources in silicon technology. For Example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon heater as an IR source made in CMOS technology, with a front side etch to suspend the heater and hence reduce power consumption. Barritault et. al “Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications” (Sensors & Actuators A 2011) describe a micromachined IR source based on a platinum heater. Several other such devices have also been reported.
It is also known to make NDIR sensors, for example, Fordl and Tille “A High-Precision NDIR CO2 gas sensor for automotive applications” IEEE Sensors Journal vol 6 No. 6 2006, and US2007/0102639 by Cutler et. al describe typical NDIR sensors consisting of a filament bulb as an IR source, and a thermopile based IR detector. The two are placed at the opposite ends of a small chamber where gas can enter through a semi permeable membrane (which blocks dust and IR radiation from outside). Depending on the concentration of the target gas, the amount of IR emission of a particular wavelength is absorbed within the optical path, and using the measurement from the IR detector can be used to determine the gas concentration. Most NDIR sensors also have an optical filter to allow only a small range of wavelengths to reach the IR detector so as to make it specific for the gas that absorbs that wavelength.
Other disclosures, such as US2008/0239322 by Hodgkinson et. al., U.S. Pat. No. 7,244,939 by Stuttard et. al, US2008/0308733 by Doncaster et. al., and U.S. Pat. No. 7,541,587 by Cutler et. al. describe similar devices.
In almost every case, the IR emitter and detector are two different components but packaged together. An exception is U.S. Pat. No. 5,834,777 by Wong, where both the emitter and detector are on the same chip with an optical path made of a waveguide by attaching (bonding) a second chip to the first chip. The process or the wafers are not CMOS, and only the waveguide is on a permeable membrane while both the emitter and the detector are outside the membrane. This will lead to high power consumption and lower maximum temperature of operation. Moreover, the optical path is relatively small for the IR emission to travel, and so the sensor has a lower sensitivity.